STEADY-STATE MODELING OF BIOGEOCHEMICAL PROCESSES IN COLUMNS WITH AQUIFER MATERIAL .1. SPECIATION AND MASS BALANCES

A laboratory column containing aquifer sand was used to study biogeoch emical processes occurring in groundwater. As a carbon source for the microbially mediated processes, lactate was introduced into the system . To describe quasi-steady-state conditions of the experiment, kinetic ally controlled biogeochemical processes were modelled together with c hemical equilibrium reactions. The laboratory column was treated conce ptually as a series of six boxes according to the position of the samp ling ports used for the chemical analysis. The model calculations focu sed on the turnover of carbon and sulfur that are related to calcium a nd iron, respectively. The comprehensive approach allows the quantific ation of all important processes and the verification of the stoichiom etries of the reactions considered. The electrons provided by lactate and its reaction products amount to approximate to 4 meq/l, which are consumed mainly by the electron accepters oxygen (0.8 meq/l), nitrate (1.3 meq/l) and sulfate (1.9 meq/l). Because all of the oxygen and nit rate was reduced in the first centimetre of the column, the formation of carbon dioxide was highly concentrated there. In the first centimet re, this led to a lower pH value (7.45) and to undersaturation with re spect to calcium carbonate. In the rest of the column, the pH was appr oximate to 7.65 and CaCO3 saturation was attained. Hydrogen sulfide, w hich was produced by ''reduction of sulfate'', reacted mainly with Fe( III) oxides in order to form FeS(s). A systematic sensitivity analysis was used to quantify the influence of the rate coefficients on the ac id-base behaviour of the system. According to these calculations, the pH is most sensitive with respect to ''disproportionation'' of lactate into propionate, acetate and bicarbonate as well as to the processes ''aerobic respiration'' and ''denitrification''.